1. Field of the Invention
The invention is concerned with polymeric coatings which are cured with particle or wave energy. A particular aspect of the invention is concerned with negative resist coatings in which patterns are generated by high energy radiation, such as, electron beam or wave energy of X-ray wavelength and shorter. Resist patterns of this nature may be utilized for protecting substrate regions from attack by a variety of agents or by other techniques used for circuit fabrication.
2. Description of the Prior Art
Fabrication of printed and integrated circuitry generally includes one or a series of processing steps in which surface regions are pattern delineated by selective material removal. Such removal, whether accomplished by use of chemical reagents, either acidic or basic, or by machining, for example, ion milling, utilizes masks or "resists". Such resists, generally adherent to the surface being processed, in accordance with this procedure define a negative image of the pattern to be delineated.
In accordance with general practice to date, resist patterns are generated in light sensitive coatings by flooding with ultraviolet light through a master mask in contact with or spaced from the coating, followed by development in an agent which differentiates between exposed and unexposed material. This procedure has been successfully utilized in the fabrication of patterns with reasonably low defect count and resolution as high as tens of microns separated by dimensions of the same order. See Handbook of Thin Film Technology, L. I. Marssel and R. Glang, Chapter 7 "Generation of Patterns in Thin Films" by R. Glang and L. V. Gregor, McGraw-Hill, N.Y. 1970. Resolution obtainable by light flooding is limited by a number of factors which diffraction and aberrations characteristic of conventional optical systems are examples. It has been recognized for some time that improved resolution would likely require development of a different system. Of the various systems investigated, the most promising at this time involves resist patterns generated by electron beams, modulated, for example, electrically and made to impinge directly on adherent radiation sensitive materials. The variety relevant to this discussion makes use of materials which are insolubilized by electron beam exposure. Alternative systems may substitute X-ray and/or may utilize masks. Where radiation sensitive layers are polymeric, insolubilization often takes the form of cross-linking. Subsequent to exposure, unexposed regions are dissolved, for example, by immersion in a suitable reagent.
A significant class of negative resist materials sensitive to electron beam or X-ray is described in "The Preparation and Characterization of a New, Highly Sensitive, Crosslinking Electron Resist" appearing in the Journal of the Electrochemical Society Vol. 122, No. 4, pages 541-544 (1975). This is an acrylate-based polymer system, generally a terpolymer of glycidyl methacrylic, ethyl acrylate, and methyl methacrylate in which epoxy moieties are totally or partially esterified with methacrylic acid. Preparation is involved, requiring a polymerization followed by a separate esterification and requiring close control at each step. Exposure of properly deposited coatings of this material results in cross-linking at reactive sites. Sensitivity is at least in part attributed to active vinyl sites produced by esterification. This material, considered by some to be marginally feasible from a preparative and an economic standpoint is, under optimum conditions, considered capable of producing patterns of usable yield with electron beam energy of less than 10.sup.-6 coulomb per square centimeter and with a resolution of the order of 1 micrometer.